1. Field of the Invention
The present invention relates generally to charged-particle apparatus for inspection and/or review and/or metrology of semiconductor wafers and other substrates.
2. Description of the Background Art
The detected signal in electron microscopes is typically a sum of secondary electrons and backscattered electrons. In some systems, the secondary electrons and backscattered electrons are separated and sent to different detectors. In some systems, the secondary electrons are subject to a threshold energy filter, with lower energies being discarded and higher energies being counted.
The use of such a threshold energy filter with a secondary electron detector has proved to be useful in enhancing the sensitivity to certain defect types in wafer inspection. This is because the secondary electrons from a wafer have energy modulation due to wafer features.
FIG. 1 is a flow chart depicting a conventional method 100 of detecting electrons using threshold energy filtering in a defect detection system. A target area on a wafer (or other substrate of interest) is irradiated 102 with an incident electron (charged-particle) beam. Due to the irradiation of the incident beam onto the substrate, secondary electrons and/or backscattered electrons are generated 104. Prior to detection of the secondary and/or backscattered electrons, a threshold energy filter is applied 106. The threshold energy filtering prevents those electrons with energies below a threshold energy level from being detected. Only the secondary and/or backscattered electrons with energies above the threshold energy level are detected and analyzed 108.
FIG. 2 schematically depicts an example conventional scanning electron microscope (SEM) system 200. The SEM system 200 includes a multiplexer control system 250 arranged to receive a plurality of multiplexer control signals and to output a plurality of image control signals to an electron beam generator subsystem (including, for example, an electron source 202, a suppressor 204, an extractor 206, an electrostatic lens 208, a gun valve 210, an upper quadrupole 211, a lower quadrupole 212, an aperture 213, a Faraday cup 214, a Wien filter 218, a magnetic lens 219, and a lower octopole 220).
The electron beam generator subsystem is arranged to receive the plurality of image control signals and to generate an electron beam 203 that is directed substantially toward an area of interest on the specimen 222. The SEM system 200 also includes a detector subsystem arranged to detect charged particles 205 emitted from the specimen 222 to allow generation of an image from the detected charged particles, which particles may include secondary electrons and/or backscattered electrons.
The detector subsystem may include an energy filter and ground mesh 223, and detector 228. The energy filter and ground mesh 223 may be arranged to select between secondary and backscattered electrons. If a high negative potential is applied to the energy filter 223, it is likely that backscattered electrons will only reach the detector 228 since backscattered electrons typically have a much higher energy value than the secondary electrons.
Once the electrons are detected by the detector 228, an image generator (for example, including analog-to-digital circuit 234 for converting the detected signal into a digital signal, a de-multiplexer circuit 236 for separating the setup phase data and image phase data, setup frame buffer 238, image frame buffer 240, and CPU 242 for processing the image frame data, among other components) is arranged to receive the detected signal 232 and generate and/or store an image data. For example, successive image frame data may be averaged together to create the image. Alternatively, the setup frame data may be utilized to generate an image.
The SEM system 200 may be implemented so as to include a multiplexer control system 250 in a form suitable for multiplexing SEM operating parameters. The multiplexer control system 250 may include a plurality of multiplexer control blocks.